1. Field of the Disclosure
The present invention relates generally to protection of power supplies, and more specifically, relates to surge protection at the input port of switch mode power supplies.
2. Background
In a typical application, an ac-dc power supply receives an input that is between 100 and 240 volts rms at a relatively low frequency that is nominally either 50 Hz or 60 Hz from an ordinary ac electrical outlet. The ac input voltage is usually rectified within the power supply to become a source of unregulated dc voltage for use by a dc-dc switching power converter. Switches in the power converter are typically switched on and off at a relatively high frequency (tens or hundreds of kilohertz) by a control circuit to provide a regulated output that may be suitable for operating an electronic device, or for charging a battery that provides power to an electronic device.
Many off-line household or industrial appliances require a regulated direct current (dc) source for their operation and often utilize different types of ac/dc switch mode power supplies to convert low frequency (e.g., 50 Hz or 60 Hz) alternating current (ac) voltage at input mains to a regulated dc output voltage or current at the output of the power supply for load operation. Switch mode power supplies are popular because of their small size due to high frequency operation, high efficiency, well-regulated outputs and the safety and protection features that are provided. One of most important protections for the ac/dc or as called the off-line power supplies due to their exposure to the power line surges that could destroy the power supply and the sensitive loads coupled to their output is the surge protection.
In general, a switch mode power supply includes a switching element accompanied with an energy transfer element, such as for example a high frequency transformer that transforms the input voltage level to a usually lower output voltage level and can serve as safety isolation as well. The output of the transformer is then rectified and filtered to provide a regulated dc output voltage or current to be provided to load.
An important recent application that utilizes switch mode power supplies is the lighting of high efficiency light sources. Such example applications include electronic ballasts for fluorescent or other discharge lamps, LED drivers and so on. LEDs have been very popular due to their longer life and their recent performance improvements in light spectrum (color), Volt-Ampere characteristics and Lumens per Watt delivery.
Safe and efficient LED drivers have recently been of great attention in the lighting industry. Off-line LED drivers can be exposed to ac power network surge pulses due to lightning or other interruptions. One safety feature that can be provided in LED drivers is surge protection device (SPD) at the input stage of the power supply. SPDs can be of different types, such as metal oxide varistors (MOVs), transient voltage suppression (TVS) diodes, thyristor surge protection devices (TSPDs) or so on.
Common input stages and interfaces included in ac-dc power supplies with the 50/60 Hz power networks include a diode bridge rectifier followed by a filter capacitance. To filter out the low frequency 120 Hz ripple of the full wave rectified input voltage, a bulk electrolytic capacitor may also be included. A bulk aluminum electrolytic filter capacitor at the input of an LED driver could be a weak point that affects the lifetime and maintenance of a driver circuit. The bulk aluminum electrolytic filter capacitor could be used to suppress and limit accidental peak surges and thus could be used as a surge protector of surges of 1 kV or less. However, in order to accommodate the extended lifetsime of the LED drivers, it is preferred in circuit design to avoid any electrolytic capacitances, which generally have shorter lifespans, and replace them with small long life capacitors.
In some power supply applications, a power factor correction (PFC) controller is included the power supply in order to comply with power factor regulations. In power supplies that include the PFC controllers, the input full wave rectified sine wave is required for operation of the PFC controller. Thus, the low frequency filter capacitance is removed or replaced with a very small capacitance in order to smooth the switching noise in the input full wave rectified sine wave. With a small filter capacitance at the power supply input after the bridge rectifier, the power supply is more susceptible to very high peak surge pulses up to several kV (˜4 kV) and surge protection at a reasonable cost is not available.